In rail vehicles, the transmission of braking forces and acceleration forces occurs at the contact point between the wheel and the rail. At this point, the transmission occurs by frictional engagement by braking friction forces acting between components composed of steel. A further important friction pairing is the contact between the brake linings and the brake disk or between the brake blocks and the wheel running surface, which is decisive in the case of braking. However, the frictional conditions of these instances of frictional engagement are dependent, inter alia, on the temperature, the degree of soiling and the relative speed.
The coefficient of friction, which is dependent on the instantaneous slip between the wheel and the rail, forms a significant influencing factor. The coefficient of friction may be formed, for example, by the coefficient of sliding friction for by the coefficient of static friction f, wherein both characteristic numbers of the relationship between the static friction force or sliding friction force Ff and the acting normal force Fn denote:
  f  ≅            F      f              F      n      
The maximum of the coefficient of friction occurs during low slip. If the slip then increases further, the coefficient of friction drops again. If the slip is then increased further, heating of the wheel and of the rail may occur as a result of the ever increasing friction power which causes damp, slippery rails to be cleaned and as a result very greatly improves the coefficient of adhesion, in particular for following wheels. Ideally, an antiskid system should set the slip of the wheels precisely in such a way that a maximum coefficient of friction is present. Since a wheel set of a rail vehicle never rolls precisely in the longitudinal direction of the rails due to the conicity of the wheels, but instead always carries out small rotational movements about the vertical axis (sinusoidal running), for example the coefficient of adhesion which can be utilized in the longitudinal direction is reduced.
Furthermore, the coefficient of friction depends to a high extent, both in terms of its magnitude and in terms of its profile, on the impurities such as water, oil, oxide layers, foliage etc., which are present between the wheel and the rail, and also on the instantaneous velocity. The coefficient of friction exhibits a falling tendency as a function of the velocity. In particular, in the fall, when there are leaves lying on the rails, a lubricating film is produced there which results in the operationally required braking performance no longer being sufficient. The rail is so smooth in such a case that the braking force which can be achieved is very low and under certain circumstances wheels slip during braking.
It is possible in such a case that during braking the braking force is set to a higher value than the braking friction force which is the maximum which can be transmitted to the contact point, resulting in the wheels of one or more axles locking. In such cases it is possible for the sliding friction which occurs then to give rise to an undesired formation of flat areas and to a reduction in the braking friction forces which can be transmitted because, with a relatively high relative speed between the friction partners, the coefficient of friction drops from a maximum at a very low relative speed. In order to prevent this, antiskid systems or antiskid regulating systems are used by which, in the case of locking of a brake, the braking force at the affected axle is reduced in order to arrive approximately in the region of static friction or of optimum brake slip again.